Conventionally, insulators such as the electrical insulated wires or the like, in which the surface of the electrical wire is coated by an insulating film, are used for motors, reactors, transformers and the like. As a method for forming an insulating film on the surface of the electric wire, a dipping method, an electrodeposition method (electrodeposition coating), and the like are known. In the dipping method, for example, a rectangular conductor or the like is used as the object to be coated. By repeating a process in which the object is dipped in a coating material, pulled up, and then dried, an insulating film having an intended film thickness is formed. The electrodeposition method is a method, in which an insulating film is formed by applying direct current to: an object to be coated, which is dipped in an electrodeposition coating material (electrodeposition dispersion), and an electrode inserted into the electrodeposition coating material, to allow charged coating material particles to be precipitated on the side of the object to be coated.
The electrodeposition method draws attention because: it is easier to coat with a uniform film thickness than other methods; and an insulating film having high rust prevention ability and adhesiveness after baking can be formed. Examples of the electrodeposition dispersion used in the electrodeposition method include a polyamide acid-base mixture made of a dispersion layer of particulate polyamic acid (polyamic acid), which is a reaction product between organic diamine and chloroformyl phthalic anhydride (trimellitic anhydride chloride), and a solvent layer of water and a neutral organic solvent; and the like (see Japanese Unexamined Patent Application, First Publication No. S49-52252 (A)), for example). According to the polyamide acid-base mixture disclosed in Japanese Unexamined Patent Application, First Publication No. S49-52252 (A), if it is kept at a temperature of about 45° C. or less, it is possible to obtain storage stability of 6 months or more. In addition, the mixture can be converted to a polyamide imide film by applying to electrodepositing suitably in a continuous manner.